MOLLUSCA. 91 



to the nioditieatiotis of the locomotive organs, into Cephalopoda, 

 Gasteropoda and Pteropoda. Headless mollnsks are bivalves 

 chiefly, and are divided into Lamellibranchiata (Conchifera), 

 Brachiopoda, Tiinicata and Poljzoa. The last three classes con- 

 stitute the division Molluscoida, the others being "true mollnsks." 



The natural families of mollnsks now verging toward extinc- 

 tion, are the Rhynchonellidce, Trigo^iiadce, and Nautilidce. The 

 following have already passed away : Productidce, OrtMdce, 

 Spiriferidm, HippitritidoB, OHhoceratidm^ Ammonitldce and 

 JBelemnitidm. Lamellibranchs have superseded Brachiopods ; 

 and Gasteropods vastly outnumber Cephalopods. The extinct 

 families and genera attained their maximum more rapidly than 

 their minimum. 



The remains of shell-bearing mollusks are the most common of 

 all fossils, and afford the most complete series of "medals" for 

 the identification of strata. All the grand divisions of the snb- 

 kingdom, even to the highest, are represented in the lowest rocks. 

 Both the highest and lowest groups were abundant in the Palae- 

 ozoic; the Lamellibranchs and Gasteropods attain their climax in 

 existing seas. Fossil species number about 19,000, most of them 

 being marine. They passed their culmination in the latter half 

 of the Mesozoic ; but the largest number of species occur in the 

 Miocene. 



